During the manufacturing process for integrated circuits, lithography (which may include photolithography, e-beam lithography, or X-ray lithography) is generally utilized multiple times as part of patterning steps. The standard method of applying the resist used for lithography is a spin-on method, whereby resist is applied at or near the center of a rotating wafer, causing the resist to spread outwards across the wafer.
The fluid dynamics of spin-coating result in the formation of an “edge bead” of resist, which is located at the wafer edge and is several times as thick as the deposited layer. This edge bead can cause significant problems later in the process if not removed, including flaking which becomes a particulate and contaminant source. Edge beads which are not removed prior to resist development can remain even after a conventional wafer stripping process. It has therefore become standard procedure to remove the edge bead immediately after spin-coating the wafer with resist. This may be accomplished in several ways, including dispensing solvent at the wafer edge and/or optical resist exposure prior to resist development. The removal of the edge bead, if done correctly, leaves a clear annulus of bare silicon at the edge of the wafer. FIG. 1a illustrates an idealized EBR geometry, including prior step stack 2 on wafer 4, resist 6 covering the top 8 and edge 10 of stack 2, and bare silicon region 12 (annular in shape) on the outside edge of wafer 4. Actual resist profiles may be gradual rather than abrupt as shown in the figure. The EBR annulus is generally between 1 and 5 mm in width, and should be accurately centered on the wafer.
One desired feature in optical inspection tools is the ability to do edge-bead removal (EBR) inspection. This includes detection of the position of the ring to determine its position and centering on the wafer, as well as determining if resist removal is complete.
Visual inspection of EBR relies on a color change at the resist edge. However, this method is inaccurate due to the differing absorption spectra and color appearances of various resists. In addition, thin film and diffraction effects may alter the color appearance. Further, if the resist does not have a sharp edge, the optical diffraction effects will be weakened.
Currently used methods for edge bead removal inspection in such inspection systems as the Viper system from KLA-Tencor Corp. utilize integrated reflectance spectra from unpolarized light to locate the edge ring where the resist has been removed. This method has proven to work well for the first resist layer. However, as illustrated in FIG. 1b, actual edge profiles for multilayer structures can be considerably more complex than the idealized profile shown in FIG. 1a. Slight differences in the centering or width of the resist region cleared at each step can result in a stepped profile 16 with multiple rings 18 of varying heights and materials underlying the resist. This complicates the optical reflectance results and causes further inaccuracies in inspection.